Pokémon Change (ポケモンチェンジ, Pokémon Change) is Pokémon Trainer's down special move. It is one of only two moves that all three of the Trainer's Pokémon have in common (the other being Triple Finish). It switches between his three Pokémon - Charizard, Squirtle, and Ivysaur. If Charizard is currently in use, it will switch to Squirtle. If Squirtle is in use, it will switch to Ivysaur. If Ivysaur is in use, it will switch to Charizard. Using this move too little will result in the Pokémon's stamina being drained. Only one Pokémon needs be knocked out to score against Pokémon Trainer.
If one Pokémon is KO'd, Pokémon Trainer will summon the next Pokémon in the rotation to the revival platform. Pokémon Change cannot be used again until the starting invincibility has worn off. Pokémon Change also cannot be used in midair.
Notably, using Pokémon Change ends nearly all status effects on the Pokémon, both positive and negative. This includes super or mini size, Starman invincibility, metal form, Franklin Badges, Screw Attacks, etc. The exceptions to this rule are slowed-time, healing in progress, and Smash Ball readiness, which remains through the change. Any held item is immediately dropped.
Like Zelda's Transform, the next Pokémon to appear must be loaded from the disc before the switch can be completed. Pausing can shorten the in-game switch time (since the game loads during the pause), as can lag when online. If the switch time is artificially shortened as such, replays will temporarily freeze at the point the switch is made (since it must take the full time to load). Forcing the game to load the next character through an SD card via hacking will result in a near-instant switch time.
A technique known as "zero switching" can be performed on horizontally moving platforms; it allows a Pokémon to switch out without while leaving the player free to react immediately after the next Pokémon is switched in. To perform the technique, the player must be standing on the edge of a platform that's opposite the direction the platform is moving (such as the right edge of the Smashville platform that's moving toward the left), and initiate Pokémon Change. If done properly, the next Pokémon will appear in the air, leaving the player free to input any aerial action. The zero switch also gives Squirtle and Ivysaur two midair jumps instead of one upon reappearance. Zero switching is commonly used to avoid the high ending lag that results from switching normally.
At the start of a match, each Pokémon has 100 points of stamina. When a Pokémon is in battle, its stamina drops by half a point every second. (This equates to 3:20 minutes of stamina.) The Pokémon also loses half a point of stamina for every attack it attempts. Once a Pokémon's stamina drops to 40, its standing animation switches to signal fatigue. When the Pokémon's stamina reaches zero, its attacks drop in damage by a factor of 0.7x. Pokémon regain 0.8 points of stamina per second when not being used, and all Pokémon's stamina is increased by 1.3x when any one of them is KO'd.
In the Subspace Emissary, the stamina stat is removed, allowing exclusive usage of one Pokémon.
The concept of stamina is generally detrimental towards Pokémon Trainer's competitive usage. As a clear attempt at forcing players to utilize the group character's full array of options instead of sticking to one of the three Pokémon for an entire match, Pokémon Trainer mains are forced to learn three characters just to stay on top of the game, let alone take control of it, and are penalized for using a single form for too long. This is a clear contrast to other transforming characters, such as the Transform mechanic for Zelda and Sheik, which has no such penalties.
Description from the Instruction Booklet
Call Charizard back and swap it for a different Pokémon. Damage carries over to the next Pokémon.
Call Squirtle back and swap it for a different Pokémon. Use a Pokémon for too long and it will tire.
Call Ivysaur back and swap it for a different Pokémon. Removing a tired Pokémon from battle lets it grow strong again.
While "Pokémon Change" isn't necessarily an official term in the Pokémon universe, it does describe the act of switching a Pokémon in mid-battle. In the games it's called "switching out" and is an action that takes place before almost all other actions for that turn (the only action that can act before a switch is the move Pursuit, which is designed to counter switching). Switching Pokémon is very important in the metagame, as the benefits of bringing in an effective Pokémon against a juicy target must outweigh the risk of being hit hard, setup on, or simply outpredicted by the opponent.
In the Pokémon games, switching out a Pokémon retains most of its current status - it will be cured of minor status effects and stat modifications, but major status effects and HP will remain unchanged. This is in contrast to Brawl, where a switched-out Pokémon regains stamina and has all status effects removed.
The concept of stamina as Brawl uses it does not exist in the Pokémon games; there is no mechanic that naturally makes a Pokémon less effective the longer it remains in play, unless one counts the fact that moves have limited uses (PP), and a single pokemon used too long will eventually be unable to do anything. Even then, however, PP does not replenish while the pokemon is switched out.